The accurate measurement of temperature has become very important to many modern industrial processes. The typical industrial temperature control system relies on signal outputs generated by some sort of temperature sensing device to provide a temperature reading for a given system. The temperature reading is utilized to regulate energy input, material input, the quantity and quality of the product being produced, environmental and safety parameters, and other parameters that are critical to the manufacturing process being conducted. Set points that operate modern processing, manufacturing, and energy generating facilities often reference measured temperature. As such, the ability to accurately measure and verify a system's temperature is critical to optimize the efficiency and safety of any temperature-dependent process. However, as a basic physical quantity, like pressure, mass and time, temperature is extremely difficult to measure accurately and great difficulties arise in determining if a temperature reading being provided by a given sensor represents the true, thermodynamic temperature. The prior art lacks any teaching of a temperature measuring device which can provide a verifiably accurate reading of temperature over a given temperature range.
A second problem associated with the various temperature measuring devices of the prior art is that all known prior art devices require periodic recalibration in a calibration oven or similar device after a period of use. Such calibration necessitates the removal of the device from the system in which it is being utilized during the period of recalibration. Removal of the temperature measuring device from the system gives rise to safety and environmental risks while the downtime associated with calibration represents a significant cost in lost productivity.
As temperature can be measured in a variety of ways via a diverse array of sensors, various methods of temperature measurement utilizing a variety of generally well understood concepts and sensors exist. The various methods of measuring temperature can be broken down into several distinct categories or families of devices, each of which is based on and utilizes different scientific principles. The various families of devices include resistance thermometry devices (RTDs and thermistors), thermocouples, optical pyrometry devices such as black body emission devices and infrared radiators, bimetallic devices, liquid expansion devices, and change of state devices. The fundamental link between these distinct families of sensors is that each family infers temperature by exhibiting some change in a physical characteristic in response to a change in temperature. RTDs measure the change in the sensor's electrical resistance as its temperature changes, with the resistance rising in an approximately linear fashion with temperature. Thermistors, which are generally constructed of various ceramic semiconductor materials, exhibit a nonlinear drop in resistance with a rise in temperature. Thermocouples measure the electromotive force (EMF) between a pair of dissimilar wires. Optical devices, such as infrared sensors, infer a temperature by measuring the thermal radiation emitted by a material. Other optical devices utilize photoluminescent principles to determine temperature. Bimetallic devices measure the difference in the rate of thermal expansion between different metals. Liquid expansion devices, such as a typical household thermometer, simply measure the volume change of a given fluid in response to a change in temperature. Finally, change-of-state temperature sensors change appearance once a certain temperature is reached. Of the foregoing temperature sensors, the vast majority of devices used in industry today utilize resistance thermometry devices, thermocouple devices, or optical devices.
The prior art contains numerous examples of resistance thermometry devices. U.S. Pat. No. 4,971,452 issued to Finney on Nov. 20, 1990, teaches an RTD for measuring the temperature of the heat receiving surface of a heat exchanger. The RTD includes an RTD assembly which is welded directly to the heat receiving surface of the heat exchanger and which shields the resistance element from combustion gases and thermally isolates the resistance element from the sheath of a sheathed cable which electrically connects the RTD to its associated circuitry. U.S. Pat. No. 5,073,758 issued to Postlewait et al. on Dec. 17, 1991, shows a circuit and method for measuring resistance in an active and high temperature environment.
The prior art also contains numerous examples of thermocouples and thermoelements. U.S. Pat. No. 5,209,571 issued to Kendall on May 11, 1993, teaches a device for measuring the temperature of molten metal. The device includes a thermocouple element, a housing consisting of a heat resistant material, and a retainer member for receiving the heat resistant element. U.S. Pat. No. 5,232,286 issued to Dubreuil et al. on Aug. 3, 1993, shows a thermocouple for high temperature measurements of liquid metals, mattes and slags. The thermocouple comprises two cermet elements of dissimilar metals in which the thermoelectric circuit is closed by the medium, the temperature of which is being measured. U.S. Pat. No. 5,121,994 issued to Molitoris on Jun. 16, 1992, shows a thermocouple probe for use in an autoclave.
The prior art also contains examples of temperature measurement devices which utilize a pair of thermocouples. As an example, U.S. Pat. No. 5,038,303 issued to Kimura on Aug. 6, 1991, teaches a method and apparatus of measuring temperature using a main thermocouple and an auxiliary thermocouple connected to one leg of the main thermocouple to provide cold junction compensation. U.S. Pat. No. 5,061,083 issued to Grimm et al. on Oct. 29, 1991, teaches a temperature monitoring device composed of at least a first thermocouple and a second thermocouple.
Each of the aforementioned prior art devices utilizes an RTD or a thermocouple or, in certain instances, a pair of thermocouples; however, the prior art contains no teaching of combining an RTD with a thermocouple. Furthermore, the prior art contains no teaching of combining any other type of impedance element (capacitors, inductors, crystals, or semiconductors) with one or more thermocouples. Finally, the prior art contains no teaching of combining two or more thermoelement wires with any type of impedance element.
The prior also teaches combining two optical temperature measuring devices. U.S. Pat. No. 5,112,137 issued to Wickersheim et al. on May 12, 1992, teaches an apparatus and method for measuring high temperature ranges using black body techniques and lower temperature ranges utilizing photoluminescent techniques, both of which are optical temperature measuring techniques. Wickersheim does not teach combining two sensors from different families of sensors such as a resistance device and a thermocouple or a resistance device and an optical device or similar combinations.
A further fundamental limitation with the temperature sensing devices of the prior art is that these devices are incapable of providing a reliable check of calibration over the temperature operating range without removal of the sensor for comparison with a known calibration reference. The fundamental limitation of all of the prior art devices is that they utilize a single family of temperature measurement devices, i.e., RTDs, thermoelements, optical devices, etc. to measure temperature. Although certain prior art devices exist which use more than one temperature measurement device, such as a pair of thermocouples or a pair of optical devices, the prior art contains no teaching of combining two dissimilar devices such as a resistive or capacitive element with one or more thermoelements.
Generally, the primary failure mode or modes for one family of measurement devices is distinct from the primary failure mode for another type of measurement method. Further, the primary failure modes for different types of devices within the same family will generally differ. A sensor element has a tendency to degrade or decalibrate due to hostile service conditions or due to an extended period of use without recalibration along a primary failure mode. If this occurs, the output signal from the sensor will no longer accurately correlate with the true, thermodynamic temperature at the point of the sensor. The prior art lacks any teaching of a device which can alert the operator to drift in the output of the sensor occurring from degradation due to any of a variety of factors while continuing to provide a true thermodynamic system temperature. U.S. Pat. No. 5,176,451 issued to Sasada et al. teaches a temperature sensor utilizing a thermocouple which includes means for indicating when a short circuit occurs in the thermocouple. A critical shortcoming of Sasada is that the operator only receives an indication when a complete sensor shutdown or short circuit has occurred. The operator receives no indication or warning when the sensor begins to decalibrate or drift and thereby is no longer reading the true, thermodynamic temperature but instead is providing an erroneous system temperature.
In the event of sensor decalibration or failure, an operator is forced to utilize other sources of information to correct for the failure, decalibration, or "drift" of the sensor. When the level of decalibration or "drift" in the sensor reaches the point where it is suspected as being unacceptable, the sensor must be recalibrated or replaced. Moreover, the prior art lacks any device or method to allow the operator to determine the amount of drift. Presently, an operator is forced to "guess," based on experience, as to the level of decalibration. In summary, the only known reliable method of verifying the accuracy of modern temperature sensors over a wide temperature range is removal and independent recalibration in a calibration furnace. For many modern applications this procedure requires costly and unacceptable shutdowns and maintenance expense. System shutdowns to accomplish the calibration function also involve significant safety risks to the individuals associated with the removal of the sensor from the system. For example, in many applications where the system must continue to operate, the removal of the sensor element is dangerous, if not impossible.
There is a need in the art for a temperature measuring device and method which can provide a true verified thermodynamic temperature.
There is a further need for a device which can be recalibrated insitu, thereby obviating the need to remove the sensor from the system for calibration.
There is a further need for a device which incorporates two or more distinct families of temperature sensors, thereby greatly reducing the likelihood that each of the sensors in a given device will decalibrate in response to the same hostile service conditions or at approximately the same point in their operational lives.
There is a further need for a device which produces a data signature comprising a variety of voltage and impedance measurements obtained from the sensor.
There is a further need for a method of compiling a data signature comprising a variety of voltage and impedance measurements and analyzing the data signature to determine a verified true system temperature.